standalone: fix failing E2E test due to getting resource base URL

[artfulrobot]

Fixes E2E\Core\PathUrlTest::testPaths_getUrl

Before

It may be right to enable this test, but it is failing :

1) E2E\Core\PathUrlTest::testPaths_getUrl                                                                                                             
Failed asserting that '' matches PCRE pattern ";https://civicrm.org/licensing;".                                                                      
                                                                                                                                                      
/buildkit/civicrm-buildkit/build/so2/web/core/tests/phpunit/E2E/Core/PathUrlTest.php:168                                                              
/buildkit/civicrm-buildkit/build/so2/web/core/tests/phpunit/E2E/Core/PathUrlTest.php:33                                                               
/buildkit/civicrm-buildkit/extern/phpunit9/phpunit9.phar:2307                                                                                         

On my buildkit, it's getting: http://so2.localhost:8100/js/Common.js but it should be fetching
http://so2.localhost:8100/core/js/Common.js

Despite the Resource URL page saying:

CiviCRM Resource URL http://so2.localhost:8100/core

After

Tests pass.

@ufundo

[civibot]

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[artfulrobot]

ok I've implemented @ufundo's suggestion. I wrapped the constant in the addtrailingslashes, just in case...